For the reproduction of audio signals, in general, frequency-amplitude characteristics are made often flat or frequency-phase characteristics are made linear. Emphasis of low frequency sound was effected only by amplifying the low frequency component and although sound volume was increased at the low frequency, clearness and crispness were not able to be obtained.
In general, it is known that the auditory sense of a human being has different sensitive times, depending on the frequency of sound. As indicated in FIG. 8, the sensitive time becomes longer with decreasing frequency and shorter with increasing frequency. For this reason, even sharp transient sound is sensed as not so sharp sound, as the frequency decreases, when it is heard by ears.
Heretofore almost no compensation of the phase characteristics (with respect to the frequency) was effected in an audio signal transmitting system. As a ground therefor it is cited that the auditory sense of a human being is almost not sensitive to the phase. For example, synthesis of a fundamental wave with a third harmonic wave is as indicated in FIGS. 11A and 11B. Although the relations in the phase between the fundamental wave and the third harmonic wave are different for FIGS. 11A and 11B, it is known that the difference on the auditory sense therebetween is small.
However recently physiological and psychological researches on the auditory sense have been progressed so that it is possible to measure delay time vs. frequency characteristics of the auditory sense.
FIG. 12 shows an example of physiological experiments on the auditory sense of a cat (refer to de Boer: "Synthetic whole nerve action potentials", J. Acoust. Soc. Am., Vol. 58, No. 5, pp. 1034, Nov. 1975) and FIG. 8 shows an example of results measured by the inventors of the present invention. In either of them, the delay time is shorter for higher frequency than for lower frequency. This means that the signal of the waveform indicated in FIG. 11A is perceived in a same manner as the signals of the waveform indicated in FIG. 11B.
In a wide band audio signal system it is sufficient to take not so seriously the phase characteristics of the auditory sense described above into account. The magnitude of different frequency components remains almost unchanged, when original sounds are heard directly and when sounds outputted by a wide band signal system are heard. It is common to both the cases that the low frequency component is heard with some delay and it is unnecessary to vary specifically the phase characteristics of the signal system.
However, in a narrow band signal transmitting system, of course the high frequency component of the signal is dropped out or attenuated and reproduced sound is far from reproduction of the original sound with high fidelity. It is for this reason that middle wavelength AM radio is heard with less clearness and crispness than FM. That is, sound reproduced by a prior art narrow band signal transmitting system had a drawback that rise of the waveform is worse, as indicated in FIG. 11B, when it is made pass through the auditory sense.
In order to remove the drawback described above, the inventors of the present invention has proposed previously a circuit, in which the delay characteristics are varied over the whole audio frequency, depending on the frequency, in Japanese patent application No. 60-14496 (JP-P-62-5713A).
In the invention of the previous application described above, e.g. a circuit indicated in FIG. 10 was used in order that higher frequency components are delayed more than lower frequency components so that relative perception time difference is removed over the whole audio frequency band.
In FIG. 10, reference numerals 27 is a frequency separator; 28 is a delaying circuit consisting of a plurality of delay lines having different delay times; and 29 is an adder.
An audio signal is separated by the frequency separator 27 into N channels of frequency bands. The signal thus separated in the different frequency bands is again synthesized by the adder 29 through the delaying circuit 28. In this case, since the signal is delayed so that higher frequency components are delayed by longer delay times, frequency-delay time characteristics of the auditory sense are compensated.
However, the construction described above had a drawback that although clearness and crispness are obtained for the low frequency region, transient sound becomes sharp in the waveform in the higher frequency region and the sound is unnatural.